1. Field of the Invention
The present invention relates to an optical head used in an optical pickup device and a method of adjusting an optical path thereof.
2. Description of the Related Art
An optical pickup device generally includes an optical head having a laser diode (LD) emitting light to a recording medium and a photo diode receiving the light reflected from the recording medium.
FIG. 1 illustrates a structure of a conventional optical head disclosed in PCT International Publication No. WO 01/43126 A2. The conventional optical head includes a substrate 8, an LD 5, a mirror 6, a prism 4 having a polarized light division function, a quarter wave plate 3, an objective lens 1, and a photodetector (PD) 7. In the above structure, light emitted from the laser 5 is reflected from the mirror 6 and reflective surfaces 4a and 4b of the prism 4 and then is converted into circularly polarized light by the quarter wave plate 3. The circularly polarized light is condensed by the objective lens 1 and then reaches a recording surface of a disc D. Thereafter, the light reflected from the recording surface of the disc D is converted into linearly polarized light by the quarter wave plate 3. Next, the linearly polarized light passes through the reflective surface 4a, is reflected from a reflective surface 4c, and returns to the PD 7. A height H of the optical head is about 3 mm.
The optical head is generally assembled according to the following process. As shown in FIG. 2, the prism 4, the quarter wave plate 3, a spacer 2, and the objective lens 1 are stacked to be attached to each other, thereby completing a first assembly body 10. As shown in FIG. 3, the first assembly body 10 is combined with a second assembly body 20, which is completed by assembling the substrate 8, the LD 5, and the PD 7. Reference numeral 9 denotes a radiating plate.
However, as is observed during a test for emitting and receiving the light after the assembling process, the light frequently deviates from a designed optical path. Stacking and assembling optical elements of the optical head are precisely performed using an align mark as a zero reference. However, since a tolerance exists in manufacturing the optical elements, although the optical elements are precisely assembled, the light may deviate from the optical path. Accordingly, relative positions of the first and second assembly bodies 10 and 20 have to be adjusted when being assembled in order to compensate for the deviation of the light from the optical path. In other words, the relative positions of the first and second assembly bodies 10 and 20 have to be adjusted so that the light emitted from the LD 5 is reflected from the recording surface of the disc D and then reaches a center of the PD 7. As shown in FIG. 4, the adjustment of the relative positions of the first and second assembly bodies 10 and 20 is achieved by rotating the first assembly body 10 around a Z-axis (corresponding to the light reflected from the mirror 6) and detecting changes in a state of an optical spot focused on the PD 7. A trajectory of the optical spot focused on the PD 7 is indicated by a dashed line in FIG. 5.
However, when the assembling process is carried out with a signal sensed by the PD 7, the first assembly body 10 rotates around the Z-axis. Thus, the assembling process does not affect the optical path through which the light is transmitted, i.e., the optical path from the LD 5 to the objective lens 1. However, if the trajectory of the optical path fails to pass through the center of the PD 7 and shifts due to the manufacturing tolerance, the first assembly 10 has to horizontally move toward an X-axis or a Y-axis of FIG. 4 so that the trajectory passes through the center of the PD 7. Then, the optical path through which the light is transmitted, re-deviates during adjusting of the optical path through which the light is focused onto the PD 7. Thus, it is quite difficult to correct the light deviating from the optical path. Also, since the optical head has a structure in which the optical elements are stacked on the substrate 8, the height of the optical head becomes higher.
Accordingly, an optical head having a new structure is demanded to solve these problems.